A polyethylene resin in-mold foam molded article, which is obtained by filling a mold with polyethylene resin foamed particles and then heating them with the use of steam, has advantageous features such as freedom in shapes, lightness, and a heat-insulating property.
Various methods for producing polyethylene resin foamed particles are known.
Patent Literature 1 discloses a method for obtaining linear low-density polyethylene resin foamed particles by dispersing linear low-density polyethylene resin particles in an aqueous dispersion medium together with an organic volatile foaming agent, heating and pressurizing the mixture so as to impregnate the linear low-density polyethylene resin particles with the organic volatile foaming agent, and then releasing the linear low-density polyethylene resin particles into a low-pressure zone so as to foam the linear low-density polyethylene resin particles. The organic volatile foaming agent used as a foaming agent in this method is a foaming agent that has high foaming power.
Patent Literature 2 discloses a method for obtaining polyethylene resin foamed particles which (i) have cell diameters of not less than 250 μm, (ii) have two melting peak temperatures (i.e., a melting peak temperature on a low-temperature side and a melting peak temperature on a high-temperature side) in differential scanning calorimetry (DSC) measurement, and (iii) have a melting peak heat quantity on the high-temperature side of 17 J/g to 35 J/g by dispersing polyethylene resin particles in an aqueous dispersion medium together with carbon dioxide (dry ice), heating and pressurizing the mixture so as to impregnate the polyethylene resin particles with carbon dioxide, and then releasing the polyethylene resin particles into a low-pressure zone so as to foam the polyethylene resin particles. Carbon dioxide used as a foaming agent in this method is a foaming agent which is more environmental-compatible than the organic volatile foaming agent but has lower foaming power than the organic volatile foaming agent.
Patent Literature 3 discloses polyethylene resin foamed particles which are obtained by using, as foaming agents, water and carbon dioxide and which contain, as novel hydrophilic compounds, polyethylene glycol and glycerin.
In particular, Patent Literature 1 and Patent Literature 2 describe use of calcium stearate for neutralizing a residue of a catalyst used for polymerization of a polyethylene resin and use of an antioxidant for preventing oxidation degradation of the resin. Patent Literature 1 and Patent Literature 2 describe, as specific examples of the antioxidant, a phenol-based antioxidant (Irganox 1010) and a phosphorus-based antioxidant (Phosphite 168).
However, Patent Literatures 1 and 2 also mention that calcium stearate and the antioxidant act also as a foam nucleating agent, and therefore addition of large amounts of calcium stearate and antioxidant causes obtained foamed particles to have extremely small cell diameters, and thus results in deterioration of surface smoothness etc. of a foam molded article. On this account, Patent Literature 1 states that calcium stearate is preferably added in an amount of 20 ppm to 300 ppm. Examples of Patent Literature 1 use a polyethylene resin containing 170 ppm of calcium stearate, 250 ppm of Irganox 1010, and 750 ppm of Phosphite 168 (1170 ppm in total (a sum of Irganox 1010 and Phosphite 168 is 1000 ppm)). Patent Literature 2 states that calcium stearate or the like is preferably added in an amount of not more than 1500 ppm, especially not more than 900 ppm. Examples of Patent Literature 2 use a polyethylene resin containing 700 ppm of calcium stearate, 300 ppm of the phenol-based antioxidant, and 500 ppm of the phosphorus-based antioxidant (1500 ppm in total (a sum of the phenol-based antioxidant and the phosphorus-based antioxidant is 800 ppm)).
Patent Literature 2 states that, in an extrusion step of obtaining resin particles which is followed by a step of obtaining foamed particles, there occurs a change in melt index and melt tension in accordance with a pelletizing temperature condition etc., and that, especially in a case where a resin temperature exceeds 250° C., there occurs a resin degradation such as decomposition and cross-linking of a polyethylene resin, which results in a decline in melt index and an increase in melt tension, thereby making it impossible to obtain foamed particles with a high expansion ratio. In order to prevent such inconvenience, Patent Literature 2 describes a method for obtaining resin particles through pelletization at a resin temperature of not more than 250° C.
However, in such a case where resin particles are obtained through pelletization at a resin temperature of not more than 250° C. in the extrusion step, the polyethylene resin has a high melt viscosity, which causes a large load to be applied to an extruder. This undesirably makes it necessary to restrict a production amount of resin particles per unit time at a low level.
In a case where resin particles are produced at a resin temperature of higher than 250° C. in order to increase a production amount of resin particles per unit time, there occur a decline in melt index and an increase in melt tension as described above, which make it impossible to obtain foamed particles with a high expansion ratio. Meanwhile, in a case where a large amount of antioxidant is added in order to avoid such inconvenience, foamed particles obtained by foaming resin particles have extremely small cell diameters. As a result, there remains a problem of a deterioration of surface smoothness etc. of a polyethylene resin in-mold foam molded article.
Patent Literature 4 and Patent Literature 5 describe polyethylene resin foamed particles with a large amount of additive. Specifically, Patent Literature 4 and Patent Literature 5 describe polyethylene resin foamed particles with 0.12 parts by weight (1200 ppm) of talc which is an inorganic substance. However, since an average cell diameter of these polyethylene resin foamed particles is less than 180 μm, there remains a problem that it is difficult to obtain a polyethylene resin in-mold foam molded article having a good surface property from such polyethylene resin foamed particles.
Although Patent Literature 4 and Patent Literature 5 describe an example of polyethylene resin foamed particles that have an average cell diameter of 180 μm or larger as well, these polyethylene resin foamed particles have an open-cell ratio of 30% or more. Accordingly, a polyethylene resin in-mold foam molded article obtained from such polyethylene resin foamed particles cannot have a good surface property due to a large shrinkage etc. Moreover, there occurs a great decline in compressive strength. For these reasons, the polyethylene resin in-mold foam molded article is not practical.
Note that the average cell diameters in Patent Literature 4 and Patent Literature 5 are ones obtained in accordance with ASTM D 3576. That is, these average cell diameters are values obtained as “L/n/0.616” where L is a certain length and n is the number of cells present in the certain length L. It should therefore be noted that these average cell diameters are values each obtained by multiplying a value obtained merely as “L/n” by 1.623 (dividing a value obtained as “L/n” by 0.616).
In addition, there is another problem that a polyethylene resin in-mold foam molded article obtained from conventional polyethylene resin foamed particles undergoes surface yellowing in an in-mold foam molding step, and this surface yellowing reduces a commercial value of the polyethylene resin in-mold foam molded article. Such surface yellowing is considered to be caused by a phenol-based antioxidant added as an antioxidant. Patent Literature 6 and Patent Literature 7 describe using a phosphorus-based antioxidant in combination with the phenol-based antioxidant in order to prevent such yellowing. However, the techniques disclosed in Patent Literature 6 and Patent Literature 7 are not ones related to a resin foam molded article. Therefore, mere application of these techniques to polyethylene resin foamed particles causes problems such as extremely small cell diameters of the polyethylene resin foamed particles as described above.
Meanwhile, Patent Literature 6 and Patent Literature 7 do not disclose a technique mentioning Z-average molecular weight (Mz) of a polyethylene resin used for polyethylene resin foamed particles.
Patent Literature 8 describes a foam molded article made of an ethylene (co)polymer having a particular molecular weight distribution (Mw/Mn). However, the ethylene (co)polymer disclosed in Examples of Patent Literature 8 is one that has a large molecular weight (Z-average molecular weight (Mz) of not less than 82×104) and that is not related to a foam molded article.
Patent Literature 9 describes a foam molded article made of an ethylene copolymer having a particular molecular weight distribution (Mz/Mw), but has no specific description about Z-average molecular weight (Mz). Moreover, the foam molded article described in Patent Literature 9 is a foam molded article obtained by kneading a mixture of an ethylene copolymer and a foaming agent and then foaming it through extrusion, foaming it in an oven, or foaming it through pressing. That is, Patent Literature 9 does not describe a foamed particle obtained by foaming resin particles after impregnating them with a foaming agent. Since such different foaming methods use base resins that are utterly different in resin properties, it is difficult to apply the technique described in Patent Literature 9 to the art of foamed particles.
Patent Literature 10 describes a cross-linked foam molded article containing an ethylene copolymer having a particular molecular weight distribution (Mz/Mw), but has no specific description about Z-average molecular weight (Mz). Moreover, the foam molded article described in Patent Literature 10 is a foam molded article obtained by injection foaming or press foaming and by cross-linking. That is, Patent Literature 10 does not describe a foamed particle obtained by foaming after impregnating resin particles with a foaming agent. Since such different foaming methods use base resins that are utterly different in resin properties, it is difficult to apply the technique described in Patent Literature 10 to the art of foamed particles.
Meanwhile, Patent Literatures 11 through 13 describe Z-average molecular weight of a base resin for use in polypropylene resin foamed particles or polystyrene resin foamed particles although Patent Literatures 11 through 13 have no mention of polyethylene resin foamed particles.
However, a polypropylene resin and a polystylene resin are utterly different from a polyethylene resin in melt characteristics such as a resin melting point and a melt index, a crystalline structure, foaming conditions such as foaming temperature, and the like. It is therefore difficult to directly apply Z-average molecular weight of a polypropylene resin or a polystylene resin to Z-average molecular weight of a polyethylene resin.